First off, you won't make a brick. So you don't have too much to worry about there.

There's not a back door. I have worn out many SD cards and multiple card readers developing the code for these radios over the past 10+ years. When I'm developing my code, I make builds to the SD card, pop the card in, and use the update. Sometimes hundreds of times a week.

The bootloader is flashed at the factory by JTAG and everything after that is done using an SD card.

Andy